1. Field of the Invention
The present invention relates to semiconductor manufacturing equipment. More particularly, the present invention relates to a device for controlling the position of a cassette in which wafers are supported, after the cassette has been loaded on a cassette support.
2. Description of the Related Art
Semiconductor manufacturing equipment typically includes a robot for automatically transferring wafers one-by-one between a wafer cassette and a process chamber in which the wafers are processed. More specifically, the wafer cassette has slots for accommodating the wafers, respectively, and the wafer cassette itself is loaded onto a wafer cassette support. Each wafer is taken out of a respective slot of the wafer cassette and is transferred to the process chamber by the robot. Once the wafer is processed, the wafer is removed from the process chamber and then is returned to a slot of a wafer cassette by the robot. A detector installed in the wafer cassette support detects whether the wafer cassette is at a position which ensures that the wafers will be smoothly transferred by the robot. The detector uses a mechanical device, such as a contact switch, to physically detect for the presence of the wafer cassette at a predetermined location on the wafer cassette support.
However, a detector relying on a mechanical elements has a very limited operability. Accordingly, a wafer cassette support having a detector that can detect for the presence of a wafer cassette with a higher degree of reliability has been developed. Such a prior art wafer cassette support and detector, as well as the wafer cassette itself, will be described with reference to FIGS. 1 to 3.
The wafer cassette 2 has the general form of a box of which one side, e.g., the front, has an opening 2a through which wafers W are loaded into or are taken out of the cassette. The wafer cassette 2 also has a pair of opposing inner walls. Each of the inner walls is made up of a plurality of wafer support projections 3 spaced vertically one above the other. The wafer support projections 3 define a plurality of slots 4 sized to accommodate the wafers W, respectively. An outer peripheral part of a wafer W is supported by a pair of the wafer support projections 3 while the wafer is accommodated in one of the slots 4. The back of the wafer cassette 2 includes a pair of protrusions 5 that extend rearwardly of the cassette 2. The bottom of the wafer cassette 2 includes a horizontal protrusion 6 extending laterally (in a direction between the sides of the cassette having the protrusions 3).
The wafer cassette support 1 has a base plate 7 formed of stainless steel, aluminum or the like. Four position fixation guides 8 disposed on the base plate 7 guide the wafer cassette 2 to a certain position on the base plate 7. To this end, the upper portion of each position fixation guide 8 has a bevel 8a that easily guides the wafer cassette 2 as the cassette 2 is lowered onto the base plate 7. The bevels 8a also serve to prevent the wafer cassette 2 from being damaged as the wafer cassette 2 is placed on the base plate 7.
As shown best in FIG. 3, the position fixation guides 8 are adapted to respectively guide an inner wall surface F1 of a front left end part of the wafer cassette 2, an inner wall surface Fr of a front right end part of the wafer cassette 2, an inner wall surface R1 of a back left end part (left side protrusion 5) of the wafer cassette 2, and an inner wall surface Rr of a back right end part (right side protrusion 5) of the wafer cassette 2. As is clear form the figure, this arrangement of the position fixation guides 8 does not compromise the space efficiency of the base plate 7.
A cassette retainer 9 is disposed at the center of the base plate 7. The cassette retainer 9 defines a guide recess 10 sized to receive the horizontal protrusion 6 (FIG. 2) at the bottom of the wafer cassette 2. Movement of the wafer cassette 2 in forward and backward directions is restrained by the cassette retainer 9 when a wafer cassette 2 rests on the base plate 7 with the protrusion 6 received in the recess 10. On the other hand, the wafer cassette 2 can slide laterally as guided by the position fixation guides 8 while the protrusion 6 is received in the recess 10.
Two of the position fixation guides 8S disposed diagonally across from each other, namely the position fixation guides for guiding the front left end part of the wafer cassette 2 and the back right end part of the wafer cassette 2, each have a slit 11 through which light can pass. The slit 11 is in the form of a tunnel extending through the position fixation guide 8S and open at the lower surface of the position fixation guide 8S. Optical detection switches 12 are provided adjacent the two position fixation guides 8S, respectively. Each switch 12 is adapted to check whether a wafer cassette 2 is present at a certain position on the base plate 7.
In particular, each optical detection switch 12 includes a light emitting part 12A for emitting visible light, for example, and a light receiving part 12B for receiving light emitted by the light emitting part 12A. The light emitting part 12A may comprise a light-emitting diode (LED) or a laser diode, and the light receiving part 12B may comprise a photo diode (PD). The light emitting part 12A and the light receiving part 12B face each other across the slit 11 of a position fixation guide 8S. Thus, if a wafer cassette 2 rests on the base plate 7 but is not present at a predetermined position relative to the base plate 7, light from a light emitting part 12A passes through the slit 11 of a position fixation guide 8S and reaches the light receiving part 12B. On the other hand, if a wafer cassette 2 is present at the predetermined position relative to the base plate 7, the light from a light emitting part 12A passes through the slit 11 but is blocked by the wafer cassette 2, i.e., the light does not reach the light receiving part 12B. Also, when a gap exists between the wafer cassette 2 and the base plate 7 after the wafer cassette 2 has been loaded onto the base plate 7, light from a light emitting part 12A passes through a slit 11, under the wafer cassette 2 and then impinges the associated light receiving part 12B. Thus, the wafer cassette 2 is detected as not being at a predetermined position on the base plate 7.
Furthermore, beneficially, the light emitting part 12A is disposed adjacent the side of the position fixation guide 8S which is not used to guide the wafer cassette 2 onto the base plate 7. Conversely, the light receiving part 12B is disposed at the side of position fixation guide 8S having the bevel 8a, namely, the side used to guide the wafer cassette 2 onto the base plate 7, as spaced therefrom. In the case in which the light emitting part 12A comprises an LED, for example, light from the light emitting part 12A is not scattered and passes through the slit 11 with good efficiency because the light emitting part 12A is disposed close to the position fixation guide 8S.
Regardless of the precision offered by the optical detectors 12, the four position fixation guides 8 are fixed. Furthermore, the position fixation guides 8 do not abut the wafer cassette 2 when the cassette 2 is supported by the base plate 7; rather, gaps are present between the external walls of the cassette 2 and the position fixation guides 8 to allow for some latitude in the operation of setting the cassette 2 on the base plate 7. Therefore, the position of the wafer cassette 2 can change or shift whenever the wafer cassette 2 is placed on the base plate 7. Therefore, the wafer can be broken if the wafer cassette 2 is not positioned precisely relative to the base plate 7 when a cassette station (C/S) arm of the robot loads or unloads a wafer into or from the cassette 2.